1. Field of the Invention
The present invention relates to plasticized polyvinylbutyral films. The invention also relates to the production of such films and to methods of using such films, for example, in multilayer laminated glass panes.
2. Description of Related Art
Plasticizer-containing polyvinylbutyrals long have been processed in large amounts by thermoplastic methods and in particular have been extruded to make films. Such films are distinguished by outstanding mechanical properties and are used as intermediate layers in multilayer laminated glass panes, preferably for vehicle glass panes, architectural glasses for the building sector, and bullet-proof glass panes.
One disadvantage of the polyvinylbutyrals (PVBs) is their high sensitivity to oxidation, particularly at high temperatures, in comparison with other polymers which can be used industrially, and their instability and sensitivity to hydrolysis in the presence of acids. PVBs are therefore advantageously stabilized before or during their processing, preferably by thermoplastic methods, by the addition of anti-oxidants, preferably phenols. Furthermore, a small amount of alkali metal hydroxide is often added to prevent acid hydrolysis.
Since films of plasticized polyvinylbutyrals frequently exhibit excessively strong adhesion to inorganic glasses, adhesion-reducing substances, for example, alkaline salts, such as potassium acetate, may be added to the PVB, before or during the production of the films, to reduce this adhesion. However, it is known that the addition of such alkaline anti-adhesion agents and the high melt temperatures of &gt;180.degree. C. which are to be used in the extrusion can lead to considerably discolored plasticized PVB films.
A large number of experiments and techniques which had the aim of reducing the discolorations of PVB films have therefore already been disclosed. U.S. Pat. No. 3,823,113 discloses the additional stabilization of PVB by a benzotriazole derivative. DE-C 33 47 040 and DE-A 32 28 076 describe the use of a combination of a phenolic anti-oxidant and a phosphite for PVB stabilization. However, it has been found that only poor or unsatisfactory stabilization results can be achieved by the methods described in the above-mentioned publications. In particular, PVB stabilization by means of phosphite may present problems under long-term conditions. Phosphites are known to undergo hydrolysis under the action of alkali and hence to release phosphorous acid, which may then lead to the degradation of the PVB or contribute to its degradation.
European Patent Application EP-A 0568 999 describes the stabilization of PVBs with special polynuclear phenols, and the combination of these phenols with synergistic nitrogen-, sulfur- or phosphorus-containing costabilizers. The thermoplastic processing of these PVBs leads to laminated films which exhibit substantially reduced yellowing compared with PVBs known to date. Although the laminated films thus produced represent a substantial improvement over the prior art, the optical properties of the film have to meet even higher requirements for many applications, especially in the case of multiple laminates for bullet-proof glasses.